Straits of Gibraltar no deterrent to breeding eels

Researchers, including one from the Technical University of Denmark, have proven for the first time that eels from the Mediterranean Sea do have a part to play in Europe's eel population.

Gallery 1 here

Based on their findings (published in the scientific journal Nature's Scientific Reports), eels from southern France in the Mediterranean Sea can find their way to the Atlantic Ocean, moving through the Strait of Gibraltar, to reach the Sargasso Sea to spawn alongside other eels from the rest of Europe.

“Our results provide evidence that Mediterranean countries also have an important role to play in helping to save the European eel. There has previously been speculation about whether eels in the Mediterranean could even find their way out into the Atlantic; thus the question was whether the Mediterranean eel was important for maintaining the eel population, or whether we might as well just eat them all. Well, obviously, we shouldn’t!” said Senior Researcher Kim Aarestrup of DTU Aqua (National Institute of Aquatic Resources), one of the authors of the study.

Eight eels equipped with satellite transmitters The team first fitted eight eels with small satellite transmitters to record the light, depth and temperature of the eels’ route. The data collected was sent to the researchers via satellite. After six months, the transmitters broke off from the animals and floated upwards to the water surface to be retrieved.

The researchers deduced that five of the eels had been eaten by predators in the Mediterranean. They knew this because of the change in the data pattern. For instance, if an eel is tracked near the water surface during the day, but the transmitter does not detect any light, this would mean that the eel had perished inside a predator’s stomach.

Based on the temperature and depth data from the five unfortunate eels, Aarestrup deduced that four of them were eaten by marine mammals like whales or seals. For the fifth eel, the scientists guessed that it had been eaten by a bluefin tuna. They came to this conclusion because marine mammals would not have dived for so long. In addition, the internal temperature of the predator was high, indicating the possibility that it was a bluefin tuna.

Deep beneath the flow therethrough After the six-month study, only three eels remained: one of them was 719km away from where it was tagged, but still in the Mediterranean Sea. The remaining two were in the Atlantic, 2,000 kilometres away from their tagging location, and they were presumed to have swum through the Strait of Gibraltar.

“It is both satisfying and interesting that for the first time we have been able to show that eels can migrate from the Mediterranean through the Strait of Gibraltar. […] They probably don’t like the strong current, so they change strategy, as we can see, and break their normal swimming pattern by swimming towards the bottom to avoid it, and staying there while passing through the Strait. Out in the Atlantic they then change back to their normal pattern again,” said Aarestrup.